A color picture tube includes an electron gun for forming and directing three electron beams to a screen of the tube. The screen is located on the inner surface of a faceplate of the tube and comprises an array of elements of three different color emitting phosphors. An apertured mask, called a shadow mask, is interposed between the gun and the screen to permit each electron beam to strike only the phosphor elements associated with that beam.
The shadow mask is a thin sheet of metal, such as AK steel or an iron-nickel alloy, that is contoured to somewhat parallel the inner surface of the tube faceplate. The shadow mask includes a large central apertured portion, a solid border portion surrounding the apertured portion, and a peripheral skirt portion. The skirt portion is angled from the other portions of the mask and is usually welded to a peripheral frame that supports the mask within a faceplate panel of a tube.
To make the shadow mask, a flat sheet of metal is etched to form the apertures, which are usually elongated slots or circular holes. Thereafter, the sheet is formed into the desired contour, such as spherical or biradial, and a skirt is formed by sweeping back the peripheral edge of the sheet. When a mask is cold-formed out of AK steel, a certain amount of springback occurs in the apertured portion of the mask, and the skirt flares slightly outwardly. For a 27 V tube, this skirt flare can be about 4.5.degree.. When an iron-nickel alloy, such as Invar (36% nickel-TM Reg. #63,970), is cold-formed into a mask contour, the springback and skirt flare are considerably greater than they are in the same size AK steel mask. For a 27 V tube with an Invar mask, the skirt flare is about 18.8.degree.. The springback and skirt flare in Invar masks are caused by the residual stresses that are created in the masks when they are formed into their contours. In the prior art, these residual stresses are at least partially controlled by hot-forming, instead of cold-forming, the masks. One hot-forming method is described in U.S. Pat. No. 4,536,226, issued to Ohtake et al. on Aug. 20, 1985. In the method disclosed in that patent, a fiat mask is first annealed at a temperature in the range of 1173.degree. K. to 1473.degree. K. Then, the mask is pressed into a domed contour at a temperature in the range of 298.degree. K. to 473.degree. K. This hot-forming method is expensive, because of the heating involved. Also, it has been found that small deviations in temperature across a mask, during pressing, can create random variations in the stresses across the mask, which result in unpredictable springback. Because of these disadvantages of hot-forming, there is a need to develop iron-nickel masks that can be cold-formed accurately with acceptable springback and skirt flare.